NBSIR 73-116

Study of the Local Resistance of

Conventional Plywood Subflooring to

Concentrated Load

Felix Y. Yokel

Center for Building Technology

Institute for Applied Technology

National Bureau of Standards

Washington, D. C. 20234

March 26, 1973

Final Report

Prepared for Office of Research and Technology

Office of Housing and Urban Development

Washington, D. C. 20410

NBSIR 73-116

STUDY OF THE LOCAL RESISTANCE OF

CONVENTIONAL PLYWOOD SUBFLOORING TO

CONCENTRATED LOAD

Felix Y. Yokel

Center for Building Technology

Institute for Applied Technology

National Bureau of Standards

Washington, D. C. 20234

March 26, 1973

Final Report

This report is a revision of NBS-10315

Prepared for Office of Research and Technology

Office of Housing and Urban Development

Washington, D. C. 20410

U. S. DEPARTMENT OF COMMERCE, Frederick B. Dent, Secretary

NATIONAL BUREAU OF STANDARDS, Richard W. Roberts, Director

Study of the Local Resistance of Conventional Plywood

Subflooring to Concentrated Load*

by Felix Y. Yokel

Center for Building Technology

National Bureau of Standards

IVashington, D. C. 202 34

Five conventional plywood floor systems, constructed in

accordance with the requirements of the FHA "r?inimum Property

Standards" were tested under concentrated loads in order to

compare their performance with that stipulated by performance

criteria developed on the basis of anticipated occupancy loads.

In 24 out of 26 tests the performance of the floor systems

exceeded that required by the criteria. Data on failure loads,

load- deflection characteristics and failure modes are presented

and discussed.

Key Words : Concentrated-Load Capacity, Evaluation Criteria,

Floors, Hardboard, Load Capacity, Operation

BREAKTHROUGH, Performance Criteria, Plvwood,

Plywood Subfloorinc^ , Subflooring, Underlayment , Wood-Frame Construction.

^Research Sponsored by the Office of Research and Technology,

Department of Housing and Urban Development.
SI Conversion Units

In view of the present accepted practice in this country for

building technology, common US units of measurement have been

used throughout this paper. In recognition of the position

of the United States as a signatory to the General Conference

on VJeights and Measures, which gave official status to the

metric SI system of units in 1960, assistance is given to the

reader interested in making use of the coherent system of SI

units by giving conversion factors applicable to US Units used

in this paper.

Length

1 in = 0.0254 meter (exactly)

Force

1 lb (Ibf) ---- 4.44 8 Newton (N)

Pressure

Ipsi = 6895 N/m^

Temperature

5/9 (Temperature °F -32) = Temperature °C

ii
TABLE OF CONTENTS

Page

1. Introduction i

1.1 Purpose of Study i 1.2 Background Information i

2. Scope of Testing Program 6

3. Test Specimens 7

3.1 Materials. 7

3.2 Description of Specimens g

4. Testing Procedure ...... n

5. Test Results 13

6. Interpretation of Test Results . 15

6.1 Compliance with the Criterion 15

6.2 Failure Modes 18

6.3 Effect of Test Location on Load Capacity and

Stiffness 21

6.4 Relative Stiffness of Floor Systems. 23

7. Conclusions 23

8 . Acknowledgement 24

9. References 25

iii

1. Introduction

1.1 Purpose of Stud/

Tliis study wa- conducted as part of an effort to develop

and improve evaluation criteria for industrialized housin;^.

The criteria were used to guide the development and evaluation

of prototype housing for the Department of Housinq and Urban

Development's Operation BREAKTHROUGH.

The subject of this study are requirements for the resistance of floors to concentrated load. The objective of the

study is to determine the level of performance of conventional

floor systems and compare their performance with that required

in the evaluation criteria for Operation Breakthrough [ 1 ] —'^

, which were based on anticipated occupancy loads.

1.2 Background Information

1.2.1 The need for Evaluate the Structural Performance of

Floors Under Concentrated Load

Present U.S. building codes and design standards for residential construction provide for floor capacitv under distributed load. The only U.S. recommendation related to concentrated loads acting on floors is contained in n nerfornance

standard by IIHFA [2] which is advisory and not enforcoable.

The standard recommends deflection limitations under a 250- lb

concentrated load, and an "exten.led- load capacity" of 450 lb

—'''pigures in brackets indicate literature references
with a residual deflection not to exceed 25 percent of the

maximum deflection. The concentrated loads are to be applied

over a 1-inch diameter area.

The lack of enforceable provisions for concentrated-load

capacity is not attributable to a lack of need for such provisions. It is merely brought about by the fact that codes are

based on conventional building systems, which by and large tend

to perform in a manner accpetable to the user under conditions

of normal use. On the other hand it is envisioned that some

innovative systems may comply with code provisions for distributed loads, but exhibit unsatisfactory performance under other

types of occupancy load. It is therefore necessary to evaluate

these innovative systems under various types of loading generated by occupancy, including critical concentrated loads.

1.2.2 Occupancy-Generated Concentrated Loads Acting on Floors

Concentrated loads on floors may be caused by heavy furniture or by human activity. Two critical conditions are identified:

1. A concentrated load of critical magnitude that may cause

damage to the entire floor, or more likely to a section

of the floor, by exerting excessive bending moments and/or

excessive shear.

2. A load that is concentrated over a very small area, thereby

causing failure by excessive compressive stress and/or

excessive punching shear.
Typical heavy concentrated loads have been studied by Boyd

[3] and are summarized below:

1. A person carrying a heavy load 350-450 lb

2. A crowded sofa (per front caster) 300-350 lb

3. An upright piano (1 caster) 200 lb

4. A grand piano (1 caster) 280 lb

5. Transportation of an upright piano (per wheel) . 250- 350 lb

6. Transportation of a grand piano (per wheel) .... 350-450 lb

Boyd concluded that since the use of grand pianos is relatively rare, the following design-loads should be used:

(a) 400 lb for several seconds

(b) 350 lb for 1/2 hour

(c) 200 lb indefinitely.

In extreme cases some casters may spread these loads over an

2

area as small as 0.5 m . Critical loading caused by load concentration over a small

bearing area is also caused by stiletto heels. Even though

these heels may no longer be fashionable, their future use

cannot be ruled out. A study of typical stiletto -heel pressures

[4] indicates a range of compressive stresses from 550 psi to

1390 psi, and one extreme value of 2,260 psi. Values of

punching shear computed from these data range from SO lb/ in to

117 lb/in. The case that produced the 2260-psi compressive

stress produced a punching shear of 156 lb/in.

3
1.2.3 Discussion of Evaluation Criteria for Concentrated Load

on Floors

The following criterion has been adox^ted as a guide for

Operation BREAKTHROUGH [1]:

"The structural floor should resist a 400-lb lead, applied

on a circular area of 5/8-in diameter and sustained for one

hour, without causing a residual indentation of the structural

surface in excess of 1/16 in, measured 1 hour after removal of

the load, and a 280 lb long-term sustained load, applied on a circular area of 5/8-in diameter.

If the wearing surface is of non-durable material, or if

there is a possibility that this surface may be removed during

the useful life of the structure, the floor should satisfy

(this) criterion with the wearing surface removed."

This criterion is intended to test the structural floor

and not the wearing surface. However, permanent -type wearing

surfaces which are left in place throughout the service-life

of the building may have a beneficial effect on the load capacity of structural floors which could be relied upon.

The criterion requires reasonable deflection recovery

under a 400-lb concentrated load sustained for one hour, and a 280-lb long-term sustained-load capacity. The term "sustainedload" capacity is not defined in the criterion. In this

investigation it is assumed that the intent of the criterion

is that a 280-lb load applied over a 5/8-in diameter area continuously during the service life of the structure should not

cause serious distress.

The 400-lb requirement would be in many cases associated

with the capacity to support a higher short-term load; however,

the relationship between the short-term capacity, the one -hour

capacity, and the long-term capacity would depend on the

4
material of the structural floor. As an example, this relationship is considered for the case of wood.

The following approximate capacities can be calculated

using the information in Reference [5] and assuming that capacities are interpreted in terms of maximum residual deflection

and that the residual deflections are related to flexural

strength:

30 -second capacity 4 85 lb

1-hour capacity 40 0 lb

1-year capacity 29 0 lb

On the other hand, for another material, instantaneous and

long- tern capacities may differ very little from the one-hour

capacity.

The compressive stress caused by the 400-lb load required

in the criterion is 1300 psi and the punching shear is 203

lb/in. If we compare the concentrated load,, the compressive

stress and the punching shear with the data in section 1.2.2,

it is evident that the criterion represents reasonable minimum

requirements with no substantial margin with respect to extreme

occupancy loads. However, it should be noted that some of the

extreme loads, caused by the moving of heavy furniture, could

be modified or avoided by simple precautions.

The loading requirements in the criterion differ from

existing techniques, such as the ASTM E7 2 test [6] and the

ASTM D 2394 test [7] . Both of these tests methods use a 1-in

5
diameter disc to transmit the load, while the criterion requires

a 5/8-in diameter loading area.

The E7 2 test is intended to measure the structural capacity of the system, and the D239 4 tests measure the strength of

the finished flooring. These tests, with proper choice of

load levels, could adequately evaluate most floor systems. A

problem, however, arises with floor systems that consist of a thin structural skin supported by stiffening elements. In

this case the system may perform satisfactorily under the

D2394 test, while under different support conditions the structural skin may fail by punching shear. On the other hand, in

order to generate adequate stress under a 1-in diameter disc,

the concentrated load would have to be increased to over 1000

lb, and in order to generate adequate punching shear the load

would have to be increased to at least 500 lb. These heavier

concentrated loads would be higher than the extreme concentrated

loads that actually act on the floor in service.

2. Scope of Testing Program

2 / Seven different types of plywood subflooring— were tested,

representing typical minimum construction standards presently

used. Most of the subfloring specimens tested were supported

— The structural material or surface which supports floor loads

and the finish flooring. If the subfloor material is sufficiently dense, smooth, stiff, dimensionally stable and possesses adequate bonding properties, finish flooring may be

applied directly without the use of underlayment

.

6
by wood joists of 2 x 4-in nominal size, spaced 16 in on

center. In a small number of specimens joist spacin.^s of

24 in, 20 in, 10 in and 6 in were used in order to investigate

failure modes. The small 4-in joist depth was selected, since

in all cases the joists were fully supported, and joist -

deflection and hence, joist size, was not a variable considered

in this investigation. Test loads were concentrated loads

which were increased until failure occurred. For part of the

specimens loads were applied in several cycles of unloading

and reloading. Deflections were measured near the point of

load application. The test loads were applied over circular

areas of 1 in, 5/8 in, and in a limited number of tests, 1/2 in

diameter. Table 2.1 shows the test variables and the scope of

the testing program.

3. Test Specimens

3.1 Materials

All materials were purchased from local suppliers and were

typical of those presently used in building construction. Plywoods met the requirements of Product Standard PSI-66 [8] for

softwood plywood. Dimensions and physical properties of the

different pl)'Tvoods used are shown in table 3.1.

7
3 / Hardboard underlayment— complied with Federal Standard

LLLB-810a, Type VI [9]. Dimensions and physical properties

of the hardboard used are shown in table 3.2.

4/ Wood joists are Construction Grade— Douglas Fir. Moisture content was 9.7 percent—^ and specific gravity was 0.41,--^

3.2 Description of Specimens

All the standard specimens were constructed in accordance

with the provisions in "FHA Minimum Property Standards" [15]

,

Sections 817.3 and 817.4.

Standard Specimens were constructed in small widths compared to the size of plywood sheets actually used in construction. This provided simulated conditions representing the least

strength and stiffness that the floors may be expected to

develop in service.

3.2.1 Standard Specimens without Underlayment

Figure 3.1 shows a typical specimen. The 2x4 joists

were 16 in long and were spaced 16 in on center. Plywood

3 / —Hardboard is a dense panelboard manufactured of wood fibers

with the natural lignin in the wood reactivated to serve as

a binder for the wood fibers. Underlayment is a material installed over the subfloor to provide a suitable base for the

finish flooring when the subfloor does not possess the necessary properties for direct application of the flooring.

4 / —In accordance with WCLIB Rules No. 14 [10]

5 / —Determined in accordance with ASTM D2016 [13]

6 / —Determined in accordance with ASTM D2 39 5 [14]

8
sheets, nominally 1/2 in thick, 14 in wide, and 4 8 in long,

were nailed to the top and bottom faces of the joists. The

plywood sheets were oriented with the grain of the outer plies

perpendicular to the axis of the joists. The joists were 2 in

longer than the width of the plywood sheet to give the specimens

stability under concentrated load, which was applied at the

long edge of the plywood. The plywood sheets were nailed to

the joists with 8d common nails. Three nails, spaced 6 in on

center, were used for the two outside joists. The inside joists

were nailed with two nails, spaced 10 in on center.

Standard specimens, as described in this section, were

made for three different floor systems:

7 / System A, using plywood a—

System B, using plywood b

System C, using plywood c

3.2.2 Standard Specimens with Underlayment

Figure 3.2 shows a typical standard specimen with underlayment. The two 4

8 -in long 2x4 joists vv'ere spaced 16 -in on

center. Four 12-in long by 16-in wide sections of nominally

1/2 -in thick plywood were nailed to the top and bottom faces

of the joists. Each 12 x 16-in plywood section was nailed on

each side by three 8d common nails, spaced 5 in on center. This

spacing was less than the 6-in spacing required in "FHA Minimum

Property Standards." The reduced nail spacing was chosen in

77

—'For description of plywoods refer to Section 3.1 and tabic 3.1

9
order to compensate for the fact that this specimen was only

16 inches wide, while in an actual building an 8 ft sheet would

be used, providing continuity at least at one of the two joist

supports. The 1/2-in plywood sheets were oriented with the

grain of the outer ply perpendicular to the axes of the joists.

A continuous sheet of underlayment , 16 in wide by 4 8 in long,

was nailed to the outer face of the 1/2 in plywood sheets.

This underlayment consisted of either 7/32-in thick hardboard

or 1/4-in thick plywood. The underlayment was nailed to the

1/2-in plywood sheets by 4d annular-thread nails spaced 6-in

on center.

Standard specimens with underlayment were made for four

different floor systems:

System D, using plywood d with 7/32-in hardboard

underlayment;

System E, using plywood c with 1/4-in plywood underlayment;

System F, using plywood c with 7/32-in hardboard underlayment;

System G, using plywood c with 1/4-in plywood underlayment .

3.2.3 Specimens With Other Than 16 in Joist Spacing

Several Specimens were made with other than 16 in joist

spacing. These specimens were all without underlayment and

were similar to the specimens described in Section 3.2.1 with

the exception of the joist spacing.

10
4 . Testing Procedure

The specimens were fabricated and stored in the ' laboratory

at approximately 73 °F and 50 percent relative humidity. The

tests were performed in the same laboratory.

The load was transmitted from the head of a 60,000-lb

capacity testing machine. The test setup is shown in figure

4.1. The specimen rested on the platten of the testing machine

Load v/as applied to the specimens through the end of a 6.5-in

long steel rod. The end of this rod was sharp edged and

machined to the required diameter. The steel rod was connected

to a load cell which was inserted between the upper end of the

rod and the head of the testing machine

.

8 / Deflection— was measured by a displacement transducer

(LVDT) . The transducer was connected to a base, made of a

2x4 wood member, 18 in long, that rested on three adjustable

bolts. These bolts were so spaced that the base could be

supported on the centerline of two joists on 16 in centers.

Deflections were measured to the face of a bracket which was

—The term "indentation" used in the criterion was interpreted

as a deflection of localized nature which was measured relative to two points on the surface of the floor, spaced 16 in

apart and which in some cases included a well defined indenta

tion of the floor surface, as well as a localized deflection

between two adjacent supporting joists. In the case of the

standard specimens, the measured deflections at the critical

locations were referenced to two points at the floor surface

located above the center lines of two adjacent supporting

joists

.

11
connected to the upper end of the load cell, thus measuring

the downward movement of the loading device relative to points

spaced 16 in apart and located at the surface of the specimen.

The distance between the centerline of the displacement transducer and the centerline of the loading rod was 4 in.

Deflections thus measured also included shortening of the

loading rod and the load cell. To determine the magnitude of

this effect the shortening of the rod and the load cell was

measured for loads up to 1000 lb. It was determined that the

effect of this shortening on test results was of second order

magnitude and corrections for this effect were therefore unnecessary.

Data were recorded electronically by transmitting the out

put from the displacement transducer and the load cell to an

X-Y recorder. The X-Y recorder plotted loads on the Y axis to

a scale of 100 lb per 1 in, and deflections on the X axis

to a scale of 0.1 in per 1 in. This produced a graphical

record of the data which had adequate resolution.

The load was applied at a rate of 1/2 lb/sec. Most

specimens were loaded continuously to failure, but several

specimens were subjected to cycles of unloading and reloading.

After each load increment of 100 lb these specimens were completely unloaded and reloaded to a load 100 lb greater than th>

previous load or to failure, whichever came first. This procedure left a record of instantaneous deflection recovery for

each specimen. On two specimens, a 400-lb load was maintained

12
for one hour and the specimens were then unloaded and deflection

recovery was measured after one hour. In some tests failure

occured at loads higher than 1000 lb. In these cases the load

cell which had a 1000-lb capacity was removed prior to the

completion of the test and loads were measured by the testing

machine. For these tests, only failure loads as defined in

Section 5 were recorded since the deflections at failure were

not measured.

5. Test Results

The test data which consist of a plotted load-deflection

curve for each specimen tested are summarized in table 5.1.

The first column in the table identifies the floor system, in

accordance with the list of floor systems in table 2.1. The

diameter of the loaded area is shown in the second column, the

joists spacing in the third column, and the location of the

test load in the fourth column. Test-load locations are identified as shown in figure 5.1. The other three columns identify

failure load, load causing initial structural damage, and

deflection at failure load, respectively.

The method by which these values were determined is illustrated in figure 5.2 which shows a typical load-deflection

curve. In general, specimens could be loaded to a certain

level without any sign of distress. First signs of distress,

which were usually associated with some cracking sound, can

be identified on the load-deflection curves as a drop m the

applied load which is not associated with a change in deflec13
tion. Such a drop in load is associated with a residual deflection which is roughly proportional to the magnitude of the

drop in load. The load level at which this first distress

occurred is identified in column 6 of table 5.1, and is shown

in figure 5.2. If loading was subsequently continued, most

specimens were able to support additional load increments without an appreciable change in the slope of the load-deflection

curve, until an additional drop in load occurred at a higher

load level.

The failure load in column 5 of table 5.1 identifies the

lowest load level at which a load drop of 30 lb or more occurred.

This point does not always represent the highest load that the

specimen can support. The definition of failure load is based

on the observation that a load drop of 30 lb was associated . with irrecoverable deflections approaching 1/16 in. It is

reasonable to assume that after such a drop in load most specimens would not meet the deflection-recovery requirements in the

criterion which specifies a residual deflection of less than

1/16 in, and that a clearly identifiable residual deflection

v/ould remain on all specimens after removal of the load.

The information presented in Table 5.1 is summarized in

Table 5.2, where the average loads causing failure and initial

distress are tabulated for specimens with 16-in joist spacing.

Other information that can be derived from the test data, together with plots of typical load-deflection curves, is presented in Section 6 where test results are interpreted.

14
6. Interpretation o£ Test Results

6.1 Compliance with the Criterion Adopted for Operation

BREAKTHROUGH

6.1.1 Concentrated-load capacityFigure 6.1 is a plot showing the range of load capacities

and average load capacities for specimens \\rith 16-in joist

spacing. The test data are for test locations 1 and lu in

figure 5.1 since these locations are considered critical.

Actually tests at locations 3, 4, and 2u yielded lower results,

however, in accordance with good construction practice, free

edges of plywood sheets should be blocked when 1/2 -in thick

plywood is used or tongue and croove joints should be provided

for thicker plywood sheets. Compliance '«'ith the criterion at

test locations 3, 4 and 2u is therefore not required.

The shaded rectangles in figure 6.1 show -^he range of the

failure loads and the unshaded rectangles show the range of

loads that caused initial distress. The solid and hollow

9 / circles— show the average loads at failure and initial distress respectively. Test results are plotted for loaded areas

of 5/8 in, as well as 1 in diameter. The heavy horizontal line

shows the load level -required by the criterion.

The following conclusions can be derived from figure 6.1:

1.) All specimens tested failed at load levels equal

to, or higher than that required by the criterion.

— In some cases the test results do not crver 3 ?is;nifican*

range, or only one single test was performed. In these caseonly the solid and hollow circles are shown.

15
2. ) Except for floor system E, c.ll specimens tested

showed riist sifinr. of distress at load levels

equal l-o or iiiqlier than that required by the

criterion. For system E, two out of the three

specimens tested showed first signs of distress

at load levels higher than that required by the

criterion, and the third specimen showed first'

signs of distress at a load of 390 lb.

3. ) In all cases, specimens tested by the 1-in diameter

disc had significantly greater load capacity than

specimens tested with the 5/8~in diameter disc.

The overall conclusion is, that except for one specimen in

system E, all specimens satisfied the criterion and most specimens exceeded the capacity required in the criterion by a substantial margin. It should be noted that this conclusion is

based on a test setup which uses specimens of 14 in and 12 in

width, respectively. This is a simulation representing the

least strength that a floor may be expected to develop. In an

actual building, where floors are continuous over much larger

areas, load capacities may be somewhat higher.

6.1.2 Deflection Recovery

Figure 6.2 shows the load-deflection curve for a test in

which floor system C was loaded in accordance with the requirement of the criterion. Deflections are plotted along the

abscissa, and loads along the ordinate.

16
Note that the instantaneous deflection under the 400 -lb

]oad was approximately 0.178 in. When the load was sustained

for an hour, this deflection increased by 0.012 in and when the

load was removed, there was an instantaneous deflection recovery

to a residual deflection of 0.02 in. One hour after unloading;,

the remaining residual deflection was 0.01 in. Thus the residual deflection measured for this specimen was substantially

smaller than the maximum 1/16-in (0.0625 in) residual deflection

stipulated by the criterion.

Figures 6.3 through 6.7 show deflection-recovery characteristics for floor systems A,B,C,F, and G, respectively. In all

cases the residual deflection, measured immediately after removal of the 400-lb load, was substantially less than 1/16 in.

This is taken as an indication that all these floor systems

have deflection-recovery characteristics which would satisfy

the criterion. Floor systems D and E were not tested under

cycles of unloading and reloading. The observation that loaddeflection curves for the tested specimens of floor systems D

and E tended to be linear below the 400-lb load is taken as an

indication that these systems have deflection-recovery characteristics similar to those of systems A,B,C,F, and C.

6.1.3 Sustained-Load Capacity

No long-term tests were conducted to determine the sustained-load capacity of the specimens. Some indication of the

magnitude of that capacity can be derived usin;>- the data presented in reference [5]. In accordance with these data, a

17
1-hour capacity of 400 lb would correspond to a 1-year capacity

of 290 lb and to a 30-year capacity of 265 lb.

If we define the 30-year capacity as the required sustained

load capacity, a one-hour capacity of 42 2 lb would satisfy tlie

280-lb requirement in the criterion. Of the 26 specimens tested

at load locations 1, 2 and lu, 24 exceeded this capacity. Thus

it can be concluded that the floor systems tested generally

satisfy the requirement for sustained- load capacity.

6.2 Failure Modes

Figures 6.8 and 6.9 illustrate two typical modes of failure

Figure 6.8 shows a typical failure of a specimen of floor

system A loaded over a 1-in diameter area and gives the appearance of a flexural tensile crack. Figure 6.9 shows the failure

mode of a specimen of floor system B, loaded over a 5/8-in

diameter area, which is typical for most specimens under this

loading except for specimens that were loaded over the joist

support at locations 5 and 3u. This mode of failure has the

appearance of a combination of a local shear failure (punching

shear) in the upper four plies together with a flexural tensile

failure in the lowest ply.

When test results are interpreted, some conclusions could

be drawn from a theoretical consideration of the effects of the

variation of the loaded area, the joist spacing, and the location of the applied load. The following theoretical considerations apply to loads acting at locations 1, 2 and lu:

18
1. ) Flexural stress would vary with joist spacing,

however the diameter of the loaded area would

have relatively little effect. Flexural failure

would probably occur under the loaded area.

2. ) Local (punching) shear would vary with the diameter

of the loaded area and would not vary with joist

spacing. Failure by local shear would occur close

to the perimeter of the loaded area.

3. ) Vertical compression would vary with the diameter

of the loaded area and would be independent of the

location of the loaded area and of joist spacing.

Indentations caused by vertical compression were determined

in the testing program by applying concentrated loads over the

joists at locations 5 and 3u. On this basis it was determined

that vertical compression would not be critical for the l~in

and the 5/8-in diameter loading discs. The 1/2-in diameter

disc was ruled out on the basis of tests performed at location

5 on floor system C where it caused a residual deflection in

excess of 1/16 in under a 400-lb load. Data for these tests

are shown in table 5.1.

Some conclusions about the failure mode can be drawn by

considering the effect of joist spacing and of the diameter of

the loaded area. It has already been noted in section 6.1.1

that load capacity increased with an increase in the diameter

of the loaded area. This effect, and the effect of joist

spacing are illustrated in figures 6.10 and 6.11.

19
Figure 6.10 shows the effect of joist spacing on failure

loads and load levels at which initial damage occurred in

system A. Note that there was considerable variation in strength

between individual specimens. The average values therefore

only represent approximate trends since the number of samples

used was sm.all.

For the 1-in diameter test load there was no difference in

strength between the 6-in and the 10-in joist spacing. At

these spacings failure probably occurred by punching shear.

For larger joist spacing the failure load dropped with increased spacing. This drop, together with the characteristics of

the typical failures which is shown in figure 6.8, leads to the

conclusion that these specimens probably failed by flexural

compression and tension.

For the 5/8-in diameter test load the failure load tends

to decrease with increased joist spacing between the 6-in and

the 16-in spacing. For spacings larger than 16 in the failure

load increased. This inconsistency may be attributable to the

strength variability (this sample was too small to be statistically significant). The dashed curve, which shov/s loads

causing initial damage, shows a consistent decrease of load

with increased joist spacing. Since for flexural failure the

failure load would be independent of disc-size and for local

shear the load would be independent of joist spacing, it is

concluded from figure 6.10 that for the 5/8-in 'lOading diameter

failure probably was caused by a complex combination of flexural

stresses and local shear.

20
Figure 6.11 shows the relationship between load capacity

and joist spacing for floor system B, loaded over a 5/8-in

diameter area. In this case capacity only slightly decreased

with joist spacing. The dominant failure mode for these specimens was probably local shear. Tests on specimens with other

than 16-in joist spacing were only performed for floor systems

A and B.

6.3 Effect of the Test Location on Load Capacity and Stiffness

6.3.1 Floor Systems Without Underlayment

Floor systems A, B and C were tested at 5 different locations. Locations 1 and 2 were between joists and 6 in from the

free edge of the plywood sheet. These locations differed only

in the fixity of the plywood sheet at the joist support. At

location 1 the edge of the plywood was nailed to one joist

support and the plywood was continuous over the other joist

support. At location 2 the plywood was continuous over both

joist supports. It was reasoned that location 1 should be

weaker than location 2 since there was less fixity at the

joist that supported the edge of the plywood sheet. However,

comparison of the average test results in table 5.2 indicates

that the strength at location 2 was similar to that of location

1 in systems A and B. Only system C had greater strength at

location 2. Locations 3 and 4 were at the edge of the plyv/ood

sheet between joists and represented points of least strength.

This can be seen from the data in table 5.2. Locecion 5 was

21
over the joist support, and as expected, supported much higher

loads

.

A comparison of load-deflection characteristics for tne

various loading points is shown for system A in figure 6.12.

As expected, location 5 is the stiffest. There is little

difference in stiffness between locations 2 and 1, and locations

3 and 4 also have comparable stiffness.. This is consistent

with the observation that there was no significant difference

in strength between locations 1 and 2, as well as between

locations 3 and 4.

Location 1 is considered to correspond to the most critical

condition in an installed floor since, in a properly constructed

floor, the free edge at locations 3 and 4 should be supported

by blocking.

5.3.2 Floor Systems With Underlayment

Floor systems D, E, F and G were tested at three locations.

Location lu is halfway between joists and at a point where two

free edges of the plywood sheet are covered by underlayment.

Location 2u is at a free, unsupported edge midway between

joists, and location 3u is over a joist. The test results at

these locations are shown in table 5.2. As expected, location

3u is the strongest and location 2u the weakest.

The load-deflection characteristics for these loading

points are compared in figure 6.13 for floor system E.

22
Location lu is considered to represent a simulation of tlie

most critical condition, since in accordance with "FHA Minimum

Property Standards" the free edge at location 2u should be

blocked.

6.4 Relative Stiffness of the Floor Systems

The load-deflection curves of the floor systems without

underlayment , loaded at location 1, are compared in figure

6.14. The ratio between applied load and measured deflection

at location 1 can be taken as a measure of stiffness. System

A was the least stiff. This system also had the least strength.

It should be noted, that system A does not meet the requirements set by "FHA Minimum Property Standards" since the thickness of the plywood was reduced by 1/32 of an inch by the

sanding of one surface.

The load-deflection curves of floor systems with underlayment, loaded at location lu, are compared in figure 6.15.

Again, the least stiff system (E) developed tlie least strength.

7. Conclusions

1.) Out of 26 tests performed on the specimens at the

weakest location likely to be encountered in a built floor, 24 exceeded the one-hour load capacity

stipulated in the Operation BREAKTIlROUnH criterion

for concentrated- load capacity, which is based on

anticipated occupancy loads, by a substantial mar,i;in

one test exactly satisfied the criterion and one tost

did not comply with the criterion,

23
2. ) For those tests that exceeded the one-hour load

capacity requirement, residual deflections were

generally smaller than the 1/16-in maximum stipulated

in the criterion.

3. ) On the basis of the data presented in reference [5]

,

it can be concluded that in 2 4 out of 26 tests

conducted the specimens probably met the performance

level under sustained load stipulated in tne criterion

4. ) The observed mode of failure under the 5/8-in

diameter loaded area was punching shear or a complex

combination of flexure and punching shear. Vertical

compressive stresses developed under the concentrated

load were not critical.

5. ) Load capacity under a 1-in diameter loaded area

exceeded the capacity under a 5/8-in diameter

loaded area by a substantial margin. Under a 1/2-in diameter loaded area vertical compressive

stresses caused by a 400-lb concentrated load

exceeded the compressive strength of the material.

8 . Acknowledgement

The contribution of the following persons is acknowledged.

Frank A. Rankine, James F. N. Seller and Lymus Pay ton from

the staff of the Center for Building Technology and James

Warfield, formerly on the Staff of the Building Research

Division of the National Bureau of Standards

.

24
9. References

[I] The Building Research Division Team, E. 0. Pfrang, Manager,

Guide Criteria for the Evaluation of Operation BREAKTIIROUGII

Housing Systems dated December 1970 (NTIS Accession

Numbers PB-212055, 212056, 212057, 212058).

[2] Housing and Home Finance Agency, Performance Standards,

Structural and Insulation Requirements for Houses, Washington, D.C., June 1947.

[3] Boyd, J. D. , Minimum Strength and Stiffness Necessary for

Wooden Floors in Houses, Paper No. 34, CSIRO, Division of

Forest Product Technology, Melbourne, Australia, 1964.

[4] Thorburn, II. T., Flooring Damage by Heels, Materials

Research Standards, ASTM, Vol. 2, No. 9, Philadelphia, Pa.,

September 1962.

[5] Wood, Lyman W. , Relation of Strength of Wood to Duration

of Load, Forest Products Laboratory, Madison Wisconsin,

December 1951.

[6] ASTM Designation E72-68, Conducting Strength Tests of

Panels for Building Construction, American Society for

Testing Materials, Philadelphia, Pa., 1968.

[7] ASTM Designation D 2394-69 Tests for Simulated Service

Testing of Wood and Wood-Base Finish Flooring, American

Society for Testing Materials, Philadelphia, Pa., 1969.

[8] National Bureau of Standards, Voluntary Product Standard

PS 1-66, Softwood Plywood, Construction and Industrial,

U.S. Government Printing Office, Washington, D.C., Julv

1970.

[9] General Service Administration, Federal Specification,

LLL-B-810 a, Building Board (Hardboard) Hard Pressed,

Vegetable Fiber, Washington, D.C., July 7, 1965.

[10] West Coast Lumber Inspection Bureau (WCLIB) , Rules No. 15,

Grading and Dressing Rules, Portland, Oregon, January 1,

1969.

[II] ASTM Designation D 805-63, Veneer, Plywood and other Glue>l

Veneer Construction Testing, American Society for Testing

Material, Philadelphia, Pa^. , 1963.

25
[12] U.S. Department of Commerce, Commercial Standard, CS 2 51-

63, Washington, D.C., April 1967.

[13] ASTM Designation D 2016-65, Tests for Moisture Content

of Wood, American Society for Testing Materials, Philadelphia, Pa., 1965.

[14] ASTM Designation D2395-65 T, Tests for Specific Gravity of

Wood and Wood-Base Materials, American Society for Testing

Materials, Philadelphia, Pa., 1965.

[15] Federal Housing Administration, Minimum Property Standards

for One and Two Living Units, Washington, D.C., November

1966, as subsequently amended.

26
TABLES

ii

27
TABLE 2.1

Number of Tests Performed

Joist Spacing, in 16 24 20 10 6

Diameter of

Loaded area, in 1 5/8 1/2 1 5/8 1 5/8 1 5/8 1 5/8 TOTAL

B A 12 18 6 6 6 6 6 6 6 72

0)

4-1 <n B 18 12 6 11 6 53

CO C 5 6 2 13

00c

•H D 14 7 21 u

o

o E . 14 7 21

1-H

y-i ,0 F 6 6 3

C/i G 7 7

Total No, of Tests 193

SUBFLOORING SYSTEMS:

A: 15/32-in-thick underlayment grade Southern Pine interior-type, 5-ply

pljrwood

.

B: 1/2-in-thick standard grade Southern Pine interior-type with exterior

glue, 5-ply pljHi/ood. C: 1/2-in-thick C-D grade Douglas Fir interior-type, 3-ply plywood,

a/

D: 1/2-in-thick standard grade Douglas Fir interior-type, 3-ply — plywood,

under 7/32-in-thick hardboard underlayment.

E: 1/2-in-thick plywood as in D under 1/4-in-thick pljwood underlayment.

F: 1/2-in-thick plywood as in C under 7/32-in-thick hardboard under la3nnent

.

G: 1/2-in-thick plywood as in C under 1/4-in-thick plywood underlayment.

a./ The core of this plywood was laminated giving the interior ply double thickness.

28
—

O >,

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tw -i-l in in

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cx o • to

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4-1 0) iH

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C VO 00 iv-i o o •H

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CO (U

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29
4Ja

<a

00

O

0) <u > >-• < 3

4-1 CO

CM

CM

COw

PQ<:H

<l}l O

,o•o

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a>o

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0}

COo

•H

CO

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cx)

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w>

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>-l (U

}-l 3

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COo

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0) H

30
TABLE 5.1. Test Results

Floor Diameter Spacing Location Failure Load Causing Deflection

System of of of Load Initial Structural at Failure

Loaded Area Joists Too 1- Damage Load

in in lb lb in

540 480 0 .52

700 670 0 .54

1 620 570 0 . 39 400 0 .30

565 530 Average

2 450 450 0 .34

600 460 0 .38

525 455 Average

n n 310 0 .67

x-J 210 210 0 .51

440 440 0 . 68

490 460 0 . 84 A 5/8 16 363 355 Average

15/32-in„

4 300 300 0 . 61

5 n] V 300 300 0 . 36 plywood 300 300 Average

1000 b/ 980 0 .12 a/

1000 b/ 950 0 .14 a/

5 1000 b/ 920 0 .13 a/

1000 b/ c/ 0 .08 a/

1000 b/ c/ 0 .07 a/

1000 b/ £/ 0 .08 a/

1 670 460 1 .20

430 280 0 .89

820 590 1 .32 5/8 24 640 343 Average

1044 1044 1 . 22

2 740 300 1 .14 600 600 0 .75 795 648 Average

^/ Deflection readings were taken at 1000 lb.

b/ The test was discontinued at the load level indicated.

c/ No information is available.

31
Floor Diameter Spacing Location Failure Load Causing Deflection

O y O U CTLIl of of of

Loaded Area Joists Test Damage Load

in in lb lb in

1 SAO 460 0.63

610 610 0.64

730 460 0,88

627 510 Average

540 540 0.50

2 740 450 0. 80

610 610 0.61

20 630 503 Average

1 990 890 0. 39 910 710 0.41

950 800 Average

5/8 10 1000 1000 0.37

1138 1138 0.30 a/

u . ^ / A 1010 950 0.31 a/

1027 1007 Average

1 1082 1082 0.25 a/

1372 1372 0.18 a/

6 1227 1227 Average

994 994 0.28

2 1290 c/ 0.22 a/

1122 c/ 0.22 a/

1172 c/ 0.22 a/

1145 Averagt ;

1040 640 0.58 a/

1208 1000 0.46 a/

1482 1000 0.49 a/

1 670 670 0. 36 1065 740 0.56 a/

970 860 0.54

795 700 0.43

795 795 0.42

1 16 1003 1 801 Average

1152 1000 0.48 a/

2 590 590 0. 31 800 710 0.62

590 590 0.32 i

t 783 723 Average 1

32
l''l<)or System

Diameter

of

Loaded Area

in

Spacing

of

Joists

in

Location

of

Test

Failure

Load

lb

Load Causing

Initial Structural

Damage

lb

Deflect ion at Failure

Load

in

1 845

860

530

845

860

370

0. 64

0.74

0.51

20 745 558 Average

- 2 850

1242

1264

660

1000

560

0.66

0.87 a/

0.84 a/

1 1119 740 Average

1 1788

1706

c/

c/

c/

c/

10 1747 Average

2 1662

1182

1726

1268

c/

c/

c/

c/

c/

c/

c/

1460 Average

1 1750

1740

c/

c/

c/

6 1745 Average

2 1546

1564

1508

1584

c/

c/

c/

c/

c/

c/

c/

1551 Average

B 1/2-in,

5 ply

plywood

5/8 16

895

860

825

600

795

730

790

760

290

480

425

590

446

895

660

810

600

741 Average

700

790

745 Average

290

470

360

590

428 Average

0.51

0.61

0.61

0.40

0.43

0.51

0.43

0.68

0.68

0.79

33
4

I' loor

Sy s tcm Diameter

of

Loaded Area

in

Spacing Location Fa llure Load Caxising Def 1 ec tion

of of Load Initial Structural at Failure

Joists Test Damage Load

in lb lb in

4 440 440 0.57

634 634 0. 68

16 537 537 Average

840 840 0. 14 1000 b/ c/ 0.12 a/

950 950 0. 13

5 1000 b/ 630 0. 14 a/

1000 b/ — c/ 0.11 a/

1000 b/ c/ 0.11 a/

890 780 1.29

945 790 1.29

730 640 1. 20

910 910 1.25

1 652 360 1.06

640 600 1. 08

600 600 0. 94 920 500 1. 27 24 748 640 Average

680 660 0. 81

2 670 670 1. 07

795 795 0. 97

770 770 0.92

729 7 24 Average

785 785 0. 79

1 634 634 0.59

800 630 0.90

20 740 683 Average

990 890 0.78

2 810 660 0.71

810 810 0.61

870 787 Average

940 690 0.39

650 650 0.24

1 830 550 0.31

1126 570 0.44 a/

900 900 0.25

10 889 672 Average

5/8

34
5

!•'

1 not System

1) i mil- Ler of

Loaded Area

in

Spacing

of

Joists

in

Location

of

Test

Failure

Load

lb

Load Caiisiiu; Initial Structural

Damage

lb

DfflecL Ion at Failure

Load

in

10 2

660

960

660

840

800

990

620

820

660

830

800

670

0.36

0.38

0.23

0.31

0.27

0.35

B 5/8 818 733Average

6 1 830

1012

680

1012

0.20

0.28 a/

921 846Average

2

790

810

938

975

790

810

938

830

0.17

0.18

0.32

0.29

878 842Average

1 580 540 0.31

2 770 770 0.37

3 250

380

250

380

0.31

0.62

5/8 16 315 315 Average

5 1000 b/

1000 b/

520

470

0.12 a/

j 0.13 a/

^95 Average

C 1/2-ino

3-ply

plywood

1/2 16 5 1000 b/

1000 b/

280

700

0.21 a/

0.19 a/

490Average

1 16 1 710

710

620

630

0.59

0. 37 710 625 Average 1

f. 3 420

660

350

460

0.76

0. 85 540 405 Ave rage

4 400 350 0.45

35
Floor Diameter Spacing Location Failure Load Causing Deflection

System of of of Load Initial Structural at Failure

Loaded Area Joists Test Damage Load

in in lb lb in

lu 780

675

695

680

570

620

660

480

1. 21 0.53

0. 57 0.71

5/8 16 708 583 Average

2u 568 330 c/

3u 1000 b/

1000 b/

c/

c / 0.18 a/

0.18 a/

1025 910 0.74 a/

1006 730 0.70

1002 1000 0. 71 D 1 16 lu 1008 1008 0. 70

1/2-ino

1064 1064 0. 71 985 960 0.80

3-ply 1015 945 Average

plywood 800 570 1.32

2u 640

700

660

440

570

500

1. 31 1.50

1.50

700 520 Average

1000 b/ c/ 0.09 a/

1000 b/ c/ 0.09 a/

3u 1000 b/

1000 b/

c/

c/

0.11 a/

0.11 a/

410 360 0.51

lu 542

540

542

540

U . J J 0.52

E 5/8 16 497 481 Average

1/2-ino 2u 400 390 0.73

3-ply 450 420 0.96

plywood

425 405 Average

+ 1/4-ino 3u 670 670 0. 22 plywood

880 810 0.18

775 740 Average

36
7

Kloor Diameter Spacing Location Failure Load Causing Deflection

Systems of of of Load Initial Structural at Failure

Joist Test Load

in in lb lb in

1002 1002 0.65

1104 1000 0.63 a/

890 890 0.63

lu 830

670

700

830

500

550

0.58

0.50

0.70

E 1 16 866 795Average

820 630 ] . 3] 380 380 0.63

2m 670

545

530

240

1.48

1.50

604 445Average

3u 1000 b/

1000 b/

1000 b/

1000 b/

c/

c/

c/

c/

0.16 a/

0.16 a/

0.21 a/

0.21 a/

F 5/8 16 lu 950 950 0.55

1/2-in, + 7/32-ino 890 860 0.58

920 905 Average

3-ply hardboard

plywood 2u 420

310

420

290

0.66

0.39

-+ 1/4-ino 365 355 Average

pl}A«)o4 3u 1000 b/

1000 b/

c/

c/

0.12 a/

0.12 a/

720 670 0.43

lu 860

770

680

690

0.56

0.56

783 680 Average

G 5/8 16 2u 350 300 0.55

1/2-ino 370 370 0,53

3-ply 360 335 Ave rage plywood

+ 1/4-ino 3u 1000 b/ c/ 0.20

plywood 1000 b/ — cj 0.19

37
TABLE 5.2

Summary of Average Test Results for Specimens with 16-in Joist Spacing

j/ o— in Diameter Area l-m Diameter Area

T*"loor' XIAvPT V 'C L. AOP^ 1Averpjc^e L V I— A Ci tj ^ T.nflfl J J ' AvPT;^ ap Average Load

Sy s tGTns of Failure Causing Initial Failure Causing Initial

Test Load Structural Damage Load Structural Damage

lb lb lb lb

A 1 565 530 1003 801

2 525 455 783 723

3 363 355

4 300 300

5 lOOCH- 975+

- - — - B . 1 795 745

2 760 745

3 446 428

4 537 537

5 1000+ 903+

C 1 580 540 710 625

2 770 770

3 315 315 540 405

4 400 350

5 lOOOf 495

D lu 708 583 1015 945

2u 568 330 700 520

3u loom- lOOCHE lu 497 481 866 795

2u 425 405 604 445

3u 775 740 1000+

F lu 920 905

2u 365 355

3u lOOCHG lu 783 680

2u 360 335

3u 1000+

38
FIGURES

39
I

40
f . 41
42
I o

3

5

O

2

O

4

o

(a) SPECIMEN WITHOUT UNDERLAYMENT

2u

D

3u

xl:

(b) SPECIMEN WITH UNDERLAYMENT

FIGURE 5.1 LOCATIOM OF TEST POINTS

43
FIGURE 5.2 IfJTERPRETATIOn OF TEST RESULTS

44
RANGE OF FAILURE LOADS

o<

o

_i

o

Ul

UJo

z

oo

1400

1200

1000

800

600-

400

200

5/8

ZD RANGE OF LOADS CAUSING

INITIAL DISTRESS • AVERAGE FAILURE LOAD

O AVERAGE LOAD CAUSING

INITIAL DISTRESS

5hO

B

5/8 5/8 5/8

?0

5/8

So

5/8 5/8

CRITERION

IN SEC. 1.2.3.

FLOOR

SYSTEM

DIA OF LOADED

AREA, in

FLOOR SYSTEM AND DIAMETER OF LOADED AREA

FIGUE 6.1 RAf^ AMD AVERAGES OF TEST RESULTS FOR TEST

LOCATIONS 1 AND lu

45

1
I HOUR SUSTAINED LOAD

DEFLECTION, in

FIGURE 6.2 CQ^PLIANCE OF FLOOR SYSTEM C WITH BREAKTHRaJGH

CRITERION

* See Figure 5.1

46
600-

DEFLECTION, in

FIGURE 5.3 DEFLECTIOfJ RECOVERY CHARACTERISTICS OF FLOOR SYSTEfl A

47

800r

DEFLECTION,in

FIGURE 5.5 DEFLECTION RECOVERY CHARACTERISTICS OF FLOOR

SYSTEfl C

49
800

\

\

0.1 0.2 0.3 0.4 05 0.6

DEFLECTION, in

rlGURE 5.6 DEFLECTION RECOVERY CHARACTERISTICS OF FLOOR

SYSTEfl F

50
800r

0.1 0.2 0.3 0.4 0.5

DEFLECTION, in

FIGURE 5.7 DEFLECTION RECOVERY CHARACTERISTICS OF FLOOR SYSTEM G

51
52
J-> < CO LJ

CC<

a:

LiJ ILlI

UJo

o

Si

Jj 3

<

O

Q_

>-

hCD

UJ

q:

<

CO\

lO<

ITU

>

o

Q

lUo

<

o

C9
ISOOr

1600

1400

1200

<

3

Q

UJ

I-<

I-Z

Lll O

Z

o

1000

800

600

400

200

0

IN

\ /FAILURE LOAD, Hn DISC

FAILURE LOAD, 5/8-in DISC

TIAL DAMAGE ^

5/8-ln DISC

Sin lOin

FLOOR SYSTEM: A

TEST LOCATION =

I

I6in 20 in 24 In

10 20

JOIST SPACING, in 30

FIGURE 5.10 RELATIONSHIP BETWEEN JOIST SPACING AND STRENGTH

54
lOOOr

<

3

Q

UJ

h-<

Iz

LUO

z

o

o

800

600

400

200-

0

FAIUURE L0AD,5/8-in DISC

INITIAL DAMAGE, 5/8- in DISC

10 In

FLOOR SYSTEM B

TEST LOCATION I

10 20

JOIST SPACING, in

16 in 20 in 24 in

FIGURE 6.U RELATIONSHIP BETWEEN JOIST SPACIf^ AND STRB^GTH

55
0 0.2 0.4 0.6

DEFLECTION, in FIGURE 5.12 RELATIONSHIP BETl^EN THE LOAD-DEFLECTION

CHARACTERISTICS AND THE POSITION OF THE

CONCENTRATED LOAD FOR FLOOR SYSTEM A

56
DEFLECTION.in

FIGURE 5,13 RELATIONSHIP BETWEEN LOAD-DEFLECTION CHARACTERISTICS

AND THE POSITION OF THE CONCENTRATED LOAD FOR FLOOR

SYSTEfl E

57

DEFLECTION,in

FIGURE 6.15 RELATIVE STIFFfJESS OF FLOOR SYSTEMS WITH UNDERLAYMENT

59

t ORM NBS-114A <i-/l) 2. Gov't Accession No.

U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET

1. PUBLIC AHON OR RKPORl NO.

NRSTR 21,:ll£i

3. Recipient's Accession No.

4. I l l AND SlIHTi l Ll-; 5. Publication Date Study of the Local Resistance of Conventional

Plywood Subflooring to Concentrated Load 6. Performing Organization Code

7. AirnioRCs)

Felix Y. Yokel

8. Performing Organization

NBSIR 73-116

10. I'rojecr/Task/'Work Llnit N. 4600460

9. PHRl'ORMING ORCiAN IZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

11. Contrai t/Gr ant Ni- IM-H-16-70

12. Sjnmsoring Org.an izat ion Name and Address

Office of Research 5 Technology

Department of Flousing 5 Urban Development

Washington, D. C. 20410

13. Type of Report & Period

Covered

Final

14. Sponsoring Agency Code

15. SUi^PLEMENTARY NOTES

This is a revision of NBS Report 10315

16. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here.) Five conventional plywood floor systems, constructed in accordance with the

requirements of the FHA "Minimum Property Standards" were tested under

concentrated loads in order to compare their performance with that stipulated

by performance criteria developed on the basis of anticipated occupancy loads.

In 24 our of 26 tests the performance of the floor systems exceeded that stipulated

by the criteria. Data on failure loads, load-deflection characteristics and

failure modes are presented and discussed.

17. KEY WORDS (Alphabetical order, separated by semicolons) Concentrated-load Capacity, evaluation

criteria; floors; hardboard; load capacity; Operation BREAKTHRCUGH ; perfomance

criteria; plywood subflooring; subflooring ; underlayment ; wood-frame construction.

18. AVAILABILITY STATEMENT

dUNLIMITED.

FOR OFFICIAL DISI'RIBUTION. DO NOT RELEASE

TO NTiS.

.9. Si-CURri'Y C-LASS

(THIS REPORT)

UNCL ASSll-H'D

21 !\U).

20. SECURl lY CLASS

(THIS PAGE)

UNCLASSIFIED

22. Pricei

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